Due to the development of reliable high power laser diodes and diode arrays, it is now possible to achieve higher power from all types of solid state lasers. Typical solid state lasers, such as Nd:YAG lasers, typically operate over a fairly narrow wavelength determined by a narrow band of atomic transitions. They are also limited in their temporal operation, e.g., their pulse modulation is limited. On the other hand, fiber gain media, such as rare earth doped fiber gain sources, can be operated comparatively over a wide wavelength band. As an example, Yb doped fiber sources are operative over a wavelength range of about 1060 nm to 1150 nm depending on a number of design parameters including fiber length and the application of wavelength-selective feedback. Also, because of the high gain of the fibers, they may be operated as amplifiers providing precise control over the temporal output of the laser source. Thus, rare-earth doped fiber gain sources, versus pumped solid state lasers, may be controlled in their temporal output over a wide range of pulse lengths and modulation rates.
The use of a single mode fiber for linear power scaling, i.e., to increase or up-scale the optical power, is better because of forced laser oscillation in single transverse mode. Also, fiber lasers offer a low cost, easily produce power source at selected wavelength operation for telecommunications, printing, signal detection and medical applications. The upper limits of power scaling in conventional single clad, rare earth doped monomode fibers is limited because of the numerical aperture (NA) and core size incompatibility of these single mode fibers with the beam parameters and NA of high power laser diodes and laser diode arrays. As outlined in the paper of H. Zellmer at al., entitled, "High-Power CW Neodymium Doped Fiber Laser Operating at 9.2 W With High Beam Quality", OPTICS LETTERS, Vol. 20(6), pp. 578-580, Mar. 15, 1995, to scale the pump power, a larger fiber core diameter that is adapted to the emitter dimensions of the high power laser diode or diode array. However, reduced beam quality results because an increase fiber diameter permits multimode operation.
To overcome this problem, specially configured double clad fibers have been developed where the pump radiation is launched directly into a multimode waveguide having an inner cladding surrounding a single mode core, i.e., a pump core or inner cladding which has a larger NA and large cross area which is compatible with the beam parameters of high power laser diodes or arrays. A double clad fiber, for example, comprises a single mode core, doped with a rare earth such as Yb, Nd, Er or other rare earth dopants, or combination of such dopants, such as, Er:Yb, surrounded by an inner cladding of lower refractive index material compared to the core. High output power of the fiber gain source is achieved by launching multimode pump light into the pump core of the fiber having a wavelength corresponding to the pump absorption bandwidth of the rare earth dopant in the fiber core. The pump light propagates in the multimode inner cladding and is absorbed into the active monomode core over the length of the fiber. The multimode inner cladding permits the multi-traversing of the core by the light with a wavelength corresponding to excited emission state of the doping atoms in the pump core for bring about stimulated transition of the excited atoms to a lower energy level resulting in gain for signal amplification. As a result, multimode pump light from a high power diode laser array is converted into single transverse mode power output of several watts from a single mode core of the double clad fiber. For example, for a typical 100 .mu.m by 300 .mu.m multimode pump light with a 100 .mu.m by 300 .mu.m diameter beam and a 0.47 NA, the beam may be efficiently coupled to the inner pump cladding of the fiber. The output from the fiber is a single mode beam with a 10 .mu.m diameter and a 0.1 NA. This is about a three order increase in coupled brightness.
These double clad fiber gain sources can be operated as fiber amplifiers or fiber lasers. The laser configuration employs feedback means such as in the form of a pair of reflectors or fiber Bragg gratings making it a relatively simple structure, but is limited in most cases to cw operation. The amplifier configuration has the advantage of accurate control of the temporal output of the fiber source. The output optical power of either the laser or amplifier configuration is limited by the amount of pump light that may be injected into the fiber, the optical-to-optical conversion efficiency, and the maximum power achievable before the onset of fiber degradation. For a given rare earth dopant, the theoretical conversion efficiency is around 40% to 70% and, in actual practice, similar levels of conversion efficiency have been achieved.
The output power from a single fiber gain source can be increased by pumping the fiber source from both ends or at multiple points along the length of the fiber. Also, the output power from a single fiber gain source can be increased by increasing the size of the fiber and its numerical aperture (NA). In practice, however, the size of the fiber is limited to a diameter of several 100 .mu.m and its NA is approximately 0.45. The NA is limited by availability of suitable polymers that can be employed for the outer cladding of the fiber. Of course, the output power of the fiber gain source with a given input aperture and NA can be increased by increasing the output power and brightness of the pump source for pumping the fiber medium. Typically, this can be accomplished by the use of multimode and multiemitter semiconductor laser diode arrays or laser bars as pumping sources. The format and brightness of the array or bar should be optimally matched to the etendue of the inner cladding of the fiber. Theoretically, the brightness of the source should not be detrimentally affected in accomplishing this reformatting but, in practice, the brightness is significantly lower. As an example, a typical fiber coupled laser bar providing 17 W of cw power may be coupled to a fiber pump core having 170 .mu.m by 330 .mu.m rectangular cross-section aperture and an NA of 0.45.
It is known to scale power in a fiber source by injecting pump light in both ends of the fiber source, such as exemplified in the to Huber U.S. Pat. No. 5,268,910. Also, it is known to scale to even higher output powers in fiber sources by increasing either the pump power, such as higher power semiconductor pump sources or using multiple semiconductor pump sources, or by increasing the pump efficiency, such as by decentering the active core of a double clad fiber relative to the surrounding pump (inner cladding) core or use longer fibers with periodic fiber bends to convert, in both of these cases, more of the multimodes in the pump core. See, for example, H. Zellmer at al., supra; the patent to Chirravuri et al. U.S. Pat. No. 5,287,216; and the to Delavaux U.S. Pat. No. 5,185,826. Moreover, it has been previously disclosed in work published by Lew Goldberg at al. to scale output power by connecting in series a plurality of fiber gain source stages, such as double clad fiber amplifiers. In this case, multiple fiber sources are coupled in series, and the power from the first fiber source is coupled into the second fiber source and so. Each fiber source may be pumped from one or both ends, such as through the employment of dichroic mirrors which separate between pump light, which is typically around 808 nm or 915 nm, and the output wavelength, which typically around 1.06 .mu.m or 1.55 .mu.m. However, there are two disadvantages in this type of scale power system. First, the power levels in cores of fiber sources down-line in the system may become so high causing fiber core degradation. Second, the level of coupling losses between fiber gain source stages will limit the number of stages that can be effectively coupled together. For example, assuming 20% coupling losses between multiple source stages and 10 watt single stage fiber sources, a net power gain cannot be increased above 50 watts of output power because the coupling loss will equal the power achieved in any subsequently coupled source stages. Of course, improvements in coupling efficiency between stages will result in higher power levels, but there are presently limits on how high of a coupling efficiency will be reasonably achieved in stage coupling.
Another possible approach for scaling power in serially connected fiber gain source stages is side pumping each fiber source periodically along the length of the stage fiber. This technique is illustrated for single mode fibers in the patent to5 Whitley et al.
A further approach for scaling power is employing polarization beam multiplexing (PBM) to combine a plurality of beams into a single output having different polarization modes as long as they are orthogonally polarized. This principle is known in the art, such as evident from the patent to Pantell et al. U.S. Pat. No. 5,311,525 wherein in column 14, there is a discussion of optical devices for coupling of optical energy or radiation between two different sets of polarization modes.
A still further approach for increasing power to provide a high power fiber amplifier is the utilization of a plurality of pump double clad fiber lasers operating at pump wavelengths within the pump band of the fiber amplifier as discussed in the to Huber patent U.S. Pat. No. 5,187,760 and disclosed in FIGS. 7 and 10 and in column 8, lines 29-42 and lines 48-54.
However, there is a need to enhance scaling of power via multiple fiber medium sources of different wavelengths and how this can be accomplished within the limited gain band of such sources, which is the subject of this application.
While improvements in output power can be achieved by improving the optical coupling and formatting between the laser diode array or bar sources and the input of the optical fiber media or by providing higher brightness of such laser diode pump sources or a combination thereof, it would also be advantageous to further improve the power scaling of these fiber gain medium sources, particularly double clad fiber sources, employing utilizing existing semiconductor pump sources and fiber coupling technology.
It is an object of this invention to provide optical gain media systems that provides scaling to high optical output powers, such as several tens to hundreds of watts of output power.
It is another object of this invention to produce optical power scaling system through wavelength division multiplexing (WDM), time domain multiplexing (TDM) and/or polarization beam multiplexing (PBM).
It is a further object of this invention to provide a power scaling through multiplex combining of multiple rare-earth fiber gain sources by taking advantage of their wide wavelength band and temporal operation.